One known art for controlling movement of a head rest is taught by Patent Document 1. In this art, a collision prediction sensor, a head rest moving mechanism and a control device for controlling the head rest moving mechanism are used. The collision prediction sensor can predict a back-side collision of a vehicle a desired time (e.g., 1.0 second) before occurrence of the collision. The control device actuates the head rest moving mechanism in response to a signal from the collision prediction sensor, thereby moving the head rest forwardly from a normal position. As a result, the head rest moves closer to a head of a passenger so as to prepare a burden imposed on the passenger at the time of the back-side collision.
In a control of movement of the head rest as set forth in Patent Document 1, the vehicle must have the collision prediction sensor. In another known art, a thin plate-shaped capacitance sensor disposed in the head rest is used in place of the collision prediction sensor. The capacitance sensor may detect a relative position (distance) of the head rest relative to the passenger head without contact. The control device actuates the head rest moving mechanism in response to a signal from the capacitance sensor, thereby moving the head rest such that the distance between the head rest and the passenger head can be maintained constant.
An additional known art is taught by Patent Document 2. In this art, a head rest is constructed to move up and down in response to a signal from a capacitance sensor so as to follow a head of a passenger.    Patent Document 1: JP 2004-9891 A    Patent Document 2: JP 2000-309242 A